Heat - Charcoal bluing
Hope this link works good article on the subject starts on page 30.
best to all
Tried a "charcoal" blue on a scrap piece of shotgun barrel a few weeks ago. Used the method describe in the book "Colour Case Hardening of Firearms". The book calls for a much higher temperature, 800°f in a furnace for a few hours. It came out splotchy. Any guidance on how I could improve my results?
I may try again with this method over the weekend.
The magic temp for me seems to be 850 if you are packing it in a crucible. Doing small parts like shotgun levers and trigger guards works very well in loose charcoal in a bin or tank - I used the quarter size bluing tanks from brownells. Parts that have variable areas of different mass like Winchester receivers (some areas are thick, and some areas are thin) will give you lots of trouble.........My advice is to do only small parts for a few years, before you try big parts. Colt auto slides are the worst - the front of the slide is low mass and the rear end of the slide is heavy mass - they are near impossible to do without a furnace ala the original factory way.
best to all